Colour rendering open questions and possible solutions

Transcription

1 Colour rendering open questions and possible solutions J Schanda Virtual Environments and Imaging Technologies Laboratory University of Pannonia, Hungary

2 Overview CIE Test sample method Possible expansions of the method Colour-preference, -discrimination Visual clarity CIE TC 1-33: updating to modern knowledge Back to visual investigations

3 Introductin Some fundamental terms: Colour perception Colour constancy Chromatic adaptaion Colour rendering: Effect of an illuminant on the colour appearance of objects by conscious or subconscious comparison with their colour appearance under a reference illuminant

4 Colour rendering index Measure of the degree to which the psychophysical colour of a CIE test colour sample illuminated by the test illuminant conforms to that of the same sample illuminated by the reference illuminant, suitable allowance having been made for the state of chromatic adaptation.

5 Present Test Source CCT Ref. Illum. test sample Chrom. Adapt. Test Samples method X,Y,Z X,Y,Z U*,V*,W* U*,V*,W*! E i R i R a

6 Present test sample method 1974: Publ. 13 2nd edition Definition on reference illuminant, tolerances Test samples: Von Kries chromatic adaptation transform Use the CIE 1964 UCS Scaling: WW halophosphate lamp should have an Ra of 50.

7 Post 1974 progress Trials to define a flatery/preference index Problem: The use of R a to optimize the SPD of tri-band fluorescent lamps The new SPDs produced large negative indices Solutions Non-liner scaling from 0 to 100 Other methods (e.g. rout mean square) for averaging R i -s

8 CIE TC 1-33 Try to use a colour apperance model As progress for CAM was slow: update for post 1976 basic colorimetric knowledge: New test samples Von Kries chrom.adapt.transform. Nayatani transformation Transform to D65 U*V*W* space L*a*b* space

9 Proposed test samples in CIELAB space TCS03/81,6 TCS04/72, b* TCS2/61,2 TCS05/55, TCS01/40, TCS06/51, TCS07/30,0 a* TCS08/51,0

10 Proposed calculation steps - Not accepted - visual results requested

11 CIE TC 1-62 report: Nakano Multispectral images semantic differential evaluation Principal component analysis First factor colourfullness, low correlation Second factor colour fidelity, good corelation

12 CIE TC 1-62 report: Davis & Ohno Simulation of 15 test sample appearance for 40 SPDs Question of gamut area extreme colour distortions Proposed metric: Colour Quality Scale

13 Recommendations by TC 1-62 Use MCC test samples (TC 1-33 recommendation) Consider task dependent reference illuminants for task dependent Colour Quality Metric Try CIECAM02 based colour difference formula Take preference, visual clarity into consideration

14 Experiments at Uni.Pannonia (several other lab-s are conducting experiments) Visual experiments in a double booth 3 main CCT groups 2700 K 4000 K Lamps: Incandewscent Fluorewcent RGB- and p-leds

15 Correlation (R value) between visual scaling and CIE 13.2,CIELAB and CIECAM02 based model Light source CIE 13.2 CIELAB CIECAM02 CoolWhite CFL(940) -0,39-0,42-0,29 CoolWhite CFL(840) 0,67 0,57 0,64 CoolWhite Fl., tradit. 0,69 0,50 0,75 CoolWhite Tri-band 0,49 0,47 0,60 P-LED -0,62-0,37 0,04 RGB-LED 1 0,75 0,74 0,79 RGB-LED 2 0,82 0,82 0,89

16 Colour harmony experiments

17 Simulation experiments

18 Simulation on CRT monitor Ra versus Visual scaling for two pictures illuminated with a number of sources, transformed to the same white point.

19 Further trials Colour gamut and colour discrimination CIE Workshop 2007 Request of lamp manufacturers Supplementary index Fidelity preference discrimination Optimization: colour quality efficacy Laboratory visual experiments

20 Conclusions Colour fidelity is required by those users for whom it is important that the colours of the lit environment should not get distorted. Colour preference might be the requirement of the general population who would like to feel well under the artificial lighting. One can envisage that the preference index of a lamp will be different depending for what one would like to use it (you certainly will prefer a different type of light in your bed-room and in your office). Colour discrimination is important in many manufacturing processes, and this will also be an important characterization item.